The present invention relates to a hybrid vehicle whose drive wheels are driven either by an engine or by an electrical motor, both of which are provided in parallel with each other. In this hybrid vehicle, when a certain condition is satisfied during a travel, the engine, whose power is transmitted through a torque converter and a speed change mechanism to the drive wheels, stops, and instead, the electrical motor drives the drive wheels.
Hybrid vehicles, which use an engine and an electrical motor, have been developed for fuel efficiency and cleanness of exhaust gas. For example, Japanese Laid-Open Patent Publication No. H11(1999)-132321 discloses a hybrid vehicle which comprises an engine, a first motor generator, a belt-type continuously variable transmission and a second motor generator. In this hybrid vehicle, the first motor generator is connected to the crank shaft of the engine, and the belt-type continuously variable transmission is connected through a torque converter to the output shaft of the engine. The second motor generator is connected to a power transmission system provided on the output side of the continuously variable transmission. In normal drive, the power of the engine is transmitted to the drive wheels with the rotational speed being changed by the continuously variable transmission. When the vehicle is halted, the engine is also halted. Thereafter, when the vehicle is started again, the drive wheels are driven by the second motor generator, and at the same time, the engine is started by the first motor generator. After the vehicle has been brought into motion, the mode of the drive is switched to drive the vehicle solely by the power of the engine.
Also, Japanese Laid-Open Patent Publication No. 2000-197209 discloses a hybrid vehicle. In this hybrid vehicle, the power of the engine is transmitted to the drive wheels through a torque converter and a gear-type speed change mechanism (i.e., a speed change mechanism with fixed ratios, used in a typical automatic transmission), which are connected to the output shaft of the engine. In parallel with this power transmission path, another path is provided for an electrical motor to drive the drive wheels. The vehicle comprises a controller for switching the drive mode to drive the wheels solely by the engine or to drive them by the engine and the electrical motor in correspondence to the torque ratio of the torque converter. By using the electrical motor in assistance of the engine, the vehicle has a high performance in acceleration especially for road condition that requires continuous acceleration. On the other hand, if the road condition does not require such large acceleration, then the assistance provided by the electrical motor is minimized to increase fuel efficiency.
By the way, the hybrid vehicle requires the switching of the drive mode, so this switching operation must be executed smoothly without a shift shock, which may otherwise happen when the hybrid vehicle is switched from the drive mode powered by the electrical motor (hereinafter referred to as xe2x80x9cmotor-driven modexe2x80x9d) to the drive mode powered by the engine (hereinafter referred to as xe2x80x9cengine-driven modexe2x80x9d), resulting in an abrupt rotational change and transmission torque change. Especially in a power-transmission mechanism that transmits the rotation of the engine through a torque converter and a gear-type speed change mechanism, it is important to avoid an abrupt rotational change and transmission torque change during the switching operation because the engine-driven mode is set up by actuation of frictionally engaging means (for example, clutches and brakes) in the gear-type speed change mechanism. Therefore, Japanese Laid-Open Patent Publication No. 2000-225871 discloses a drive-mode switching method, in which, at first, the rotational speeds at the input side and the output side of the speed change mechanism are brought into a mutually agreeing rotational speed, and then, by engaging a clutch, the drive mode is switched from the motor-driven mode to the engine-driven mode, in which the power of the drive mode is supplied through the speed change mechanism.
However, even though the input and output sides of the speed change mechanism are brought to an identical rotational speed, there is a possibility that the input rotational speed of the transmission differs from the output rotational speed of the engine because the torque converter exists between the output side of the engine and the input side of the speed change mechanism (i.e., the input and output of the torque converter are different in rotational speed). If the clutch is brought into engagement in this condition, it is difficult to execute the switching of the drive mode smoothly. In other words, if there is a difference between the rotational speeds of the input and output of the torque converter, then the torque transmission through the torque converter is carried out correspondingly to this rotational difference. If the drive mode is switched from the motor-driven mode to the engine-driven mode in this condition, then the switching may not be smooth because an abrupt torque change can occur and result in a shock.
It is an object of the present invention to provide a control system for a hybrid vehicle, which system is capable of switching smoothly the drive mode of the vehicle especially from motor-driven mode to engine-driven mode.
According to the present invention, a hybrid vehicle comprises an engine, a torque converter, a speed change mechanism (for example, the gear-type speed change mechanism, which comprises the transmission 10 described in the following embodiment), frictionally engaging means (for example, the shift clutches 13c and 14c in the following embodiment), wheels (for example, the wheels 6 in the following embodiment) and an electrically driven motor (for example, the second motor generator 2 in the following embodiment). In the construction of the hybrid vehicle, the engine allows to be stopped temporarily on a predetermined drive condition. The torque converter is connected to the output shaft of the engine, and the speed change mechanism is connected to the output shaft of the torque converter to change the output rotation of the torque converter. The frictionally engaging means is placed in the speed change mechanism to set a speed ratio, and the wheels are driven by the output rotation of the speed change mechanism. Additionally, the electrically driven motor can drive these wheels or other wheels. For this hybrid vehicle, the control system comprises engine-rotation control means (for example, the throttle controller TH in the following embodiment) and engagement control means (for example, the shift control valve CV in the following embodiment). The engine-rotation control means controls the rotation of the engine, and the engagement control means controls the engagement of the frictionally engaging means in correspondence to the control executed by the engine-rotation control means. When the drive mode of the hybrid vehicle is switched from motor-driven mode to engine-driven mode, the engine-rotation control means determines a target speed ratio for the speed change mechanism and calculates the vehicle-speed corresponding rotational speed (for example, the target TC input rotational speed NTCIN(O) in the following embodiment) that would arise at the output shaft of the engine if the current rotation of the drive wheels were transmitted through the speed change mechanism set at the target speed ratio and through the torque converter with a speed ratio of 1.0. Then, the engine-rotation control means controls the rotation of the engine to bring the output rotational speed of the engine closer to the vehicle-speed corresponding rotational speed. While the rotation of the engine is being controlled in this way by the engine-rotation control means, if the deviation of the output rotational speed of the engine from the vehicle-speed corresponding rotational speed becomes and remains equal to or smaller than a predetermined value (for example, 50 rpm as in the following embodiment) for a predetermined time period (for example, 0.2 seconds as in the following embodiment), then the engagement control means engages the frictionally engaging means to set the target speed ratio.
In this control system, when the drive mode is switched from the motor-driven mode to the engine-driven mode, the engine-rotation control means controls the rotation of the engine (and the input rotation of the torque converter, which is connected to the engine) to come closer to the vehicle-speed corresponding rotational speed. In this instance, little load is acting on the output side of the torque converter (i.e., little force is acting to rotate the input shaft of the speed change mechanism) because the frictionally engaging means has been disengaged for the motor-driven mode. In this condition, while the input rotation of the torque converter, which is connected to the output shaft of the engine, changes in response to the change of the engine rotation, the output rotational speed of the torque converter also changes following this input rotational change. Therefore, the output rotational speed of the torque converter also comes closer to the vehicle-speed corresponding rotational speed. While this rotational control is in progress, if the deviation of the output rotational speed of the engine from the vehicle-speed corresponding rotational speed becomes and remains equal to or smaller than a predetermined value for a predetermined time period, then the frictionally engaging means is engaged by the engagement control means to set the target speed ratio. In this way, the frictionally engaging means is brought into engagement with almost no torque transmission while the speed ratio of the torque converter is almost 1.0. As a result, the drive mode is switched smoothly to the engine-driven mode. Also, because the output rotational speed of the torque converter is controlled to come closer to the vehicle-speed corresponding rotational speed, the rotational difference across the frictionally engaging means is relatively small when the frictionally engaging means is brought into engagement. As a result, the engagement of the frictionally engaging means is executed smoothly.
It is preferable that an auxiliary electrically driven motor (for example, the first motor generator 1 in the following embodiment) be provided in connection to the engine, so that it will assist the rotation of the engine. In this case, the engine-rotation control means controls the auxiliary electrically driven motor to assist the rotation of the engine in such a way that, when the drive mode is switched from the motor-driven mode to the engine-driven mode, the output rotational speed of the engine will come closer to the vehicle-speed corresponding rotational speed. It is difficult to control the rotation of the engine if this control is done solely by the throttle opening control. On the other hand, the rotational control of the auxiliary electrically driven motor is executable accurately because it is executed by electrical power supply control. Therefore, by using the auxiliary electrically driven motor, the rotation of the engine is controlled accurately.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.